Submodule Temperature Regulation and Balancing in Modular Multilevel Converters

In modular multilevel converters (MMCs), temperature control of semiconductor devices in the submodules (SMs) is a key factor for the safe and reliable operation. Under normal operation, significant temperature differences can exist between SMs due to, for example, aging of semiconductor modules and module parameter mismatch. This paper presents a method for achieving SM thermal balancing by controlling the capacitor voltage of each SM in an arm, while maintaining the sum of the SM capacitor voltages at a constant value in order to regulate the dc-link voltage. The proposed temperature balancing strategy is validated using an experimental MMC setup with three SMs, where an increase in the thermal resistance to ambient of one or more SM semiconductors is created by restricting coolant flow to simulate a partial failure in the cooling system. Increases in the thermal resistance by 21% and 42%, corresponding to temperature increases of 5 and 10 $^{\circ}$C, respectively, are managed by three SMs, using a capacitor voltage margin of 60%.